Implantable medical devices are available for monitoring physiological signals for use in diagnosing and managing cardiac disease. For example, implantable hemodynamic monitors can monitor heart rhythm, blood pressure and thoracic fluid status for tracking the status of heart failure patients. In the early stages of heart failure, compensatory mechanisms occur in response to the heart's inability to pump a sufficient amount of blood. One compensatory response is an increase in filling pressure of the heart. The increased filling pressure increases the volume of blood in the heart, allowing the heart to more efficiently eject a larger volume of blood on each heart beat. Increased filling pressure and other compensatory mechanisms can initially occur without overt heart failure symptoms.
The mechanisms that initially compensate for insufficient cardiac output, however, lead to heart failure decompensation as the heart continues to weaken. The weakened heart can no longer pump effectively causing increased filling pressure to lead to chest congestion (thoracic edema) and heart dilation, which further compromises the heart's pumping function. The patient begins the “vicious cycle” of heart failure which generally leads to hospitalization.
Typically, therapy for a patient hospitalized for acute decompensated heart failure (ADHF) includes early introduction of intravenous infusion of diuretics or vasodilators to clear fluid retained by the patient. This therapy can be highly effective in reducing ADHF symptoms rapidly, but overdiuresis can occur if the intravenous infusion of drugs is delivered too long or at too high of a dosage. Since there is a lag in time between reaching an optimal fluid volume status and the alleviation of symptoms, determining the optimal parameters for controlling the intravenous infusion therapy remains a challenge to clinicians. Overdiuresis may require fluid to be administered to the patient to increase the patient's fluid volume status. Removing and adding fluid can pose additional burden on the kidneys, which may already be compromised due to renal insufficiency in the heart failure patient. At other times, the fluid removed may not be sufficient to achieve a desired result. A need remains, therefore, for apparatus and methods for guiding ADHF therapy.